C-13 CHEMICAL-SHIFT TENSORS OF CARBOXYLIC-ACIDS - GIAO CALCULATIONS IN ACETIC-ACID PLUS METHYLAMINE DIMER

GIAO calculations are presented of the C-13 chemical shift tensors of the carboxyl group in the dimer of acetic acid with methylamine. The c alculations indicate that the local geometrical parameters describing the carboxyl group are related linearly to the distance between the pr oton and oxygen atom of the hydroxyl group, which is also a measure of the strength of the hydrogen bond. The individual principal component s of the chemical shift tensor exhibit very different dependences on t he O-H distance. While delta(33) is almost independent of the O-H dist ance, delta(33) varies by more than 70 ppm for O-H distances ranging f rom 0.8 Angstrom to 1.5 Angstrom. The calculated values indicate that the difference between the in-plane shielding components, delta(11) an d delta(22), can be used as a measure of the relative delocalization o f the pi electrons of the carbonyl group, the strength of the hydrogen bond and the local geometry of the carboxyl group.